Field of the Invention
The invention relates to an electrical lead-through bushing with an internal conductor surrounded by an external conductor, which are led through a flange element. The invention also relates to a combination wherein the bushing is utilized.
Bushings of this general type, which are usually also referred to as “coaxial bushings” or “coaxial lead-through bushings,” are usually used to transmit electrical signals or pulses from a first space region into a second space region, which is separated from the first space region for example by a wall or shielding. The wall or shielding may thereby have in particular an opening through which the electrical lines provided for passing on the signal or pulse are led, and which can be closed by a flange element. Usually provided in this case in the region of this flange element are an internal conductor and an external conductor surrounding the latter, which pass through the flange element and consequently establish an electrical connection from a first side of the flange element to the second side of the flange element.
Such bushings may be significant in particular if the leading-in and/or out of electrical signals or pulses into the interior of an actually completely enclosed vessel is desirable or required, for example for diagnostic or monitoring purposes. An example of such an application is the so-called TDR measuring principle (TDR, time domain reflectometry), which can be used for determining or monitoring the position of a boundary layer between two media or two phases, in particular for ascertaining a fill level, in a vessel. The TDR measuring principle is described, for example, in German patent DE 199 58 584 C1 and in U.S. Pat. No. 4,786,857.
In a TDR measurement process, which is used in particular whenever the vessel cannot be inspected in some other way, and consequently the characteristic positional value of the boundary layer or of the filling level cannot be determined directly, use is made of the effect that an electromagnetic pulse carried in an antenna system is partly reflected when the impedance between, for example, a central conductor of the antenna and an external conductor surrounding the latter in the manner of a coaxial cable changes abruptly. Such an abrupt change of the impedance occurs, for example, wherever the antenna formed in such a manner is immersed from a gaseous environment into a liquid, since the impedance depends on the capacitance between the central conductor or internal conductor and the external conductor, and consequently depends on the dielectric constant of the medium filling the intermediate space between the central conductor and the external conductor. An electromagnetic pulse supplied to such an antenna which has been immersed into the medium to be monitored is consequently partly reflected at the surface of the medium. A further reflection occurs at the, usually short-circuited, end of the antenna. Since the propagation velocity of the electromagnetic pulse in the antenna is otherwise known, the difference in the delay time between the pulse reflected at the boundary layer and the pulse reflected at the end of the antenna can be used as a measure of the position of the boundary layer, and consequently as a means for ascertaining a characteristic positional value that is characteristic of the position of the boundary layer. It is thereby possible to take as a basis a substantially proportional relationship between the difference in delay time and the characteristic positional value.
To allow this method to be used for the diagnosis or monitoring of a medium in a closed vessel, for example, the transmission of electromagnetic pulses from the outside into the interior of the vessel and vice versa is consequently required. On the other hand, however, depending on the type and properties of the medium kept in the vessel, it may be absolutely necessary, or at least very important, to ensure a particularly high degree of sealing integrity of the vessel. Depending on the operating parameters prevailing in the vessel according to the design, such as for example the pressure and temperature of the medium kept there, particularly demanding requirements are consequently to be imposed in an individual case on the electrical bushing used for leading electromagnetic pulses in and out. This may have the effect that, for example, the TDR measuring principle cannot be used at all under certain circumstances and for certain media.